A copper foil for printed circuits generally is laminated and bonded to a base of a synthetic resin or the like at a high temperature and a high pressure, screen-printed with a necessary circuit pattern using a resist to form an objective circuit, and then is etched with an etchant such as hydrochloric acid to remove unnecessary portions. Finally, necessary elements are soldered in place, and in this way various printed circuit boards for electronic devices are fabricated. Qualitative requirements for the copper foil for printed circuit boards differ with the side surfaces, the surface of the side to be bonded to the resin base (matt side) and the surface of the side not to be bonded to the resin base (shiny side).
Requirements for the matt side chiefly include:
(1) No possibility of oxidative discoloration during storage (corrosion protection, rust prevention);
(2) Adequate resistance to peeling from the base even after high-temperature heating, wet treatment, soldering, chemical treatment or the like (peel strength); and
(3) Freedom from so-called lamination spots that can be observed after lamination to the base and etching (hydrochloric acid resistance).
Requirements for the shiny side include:
(1) Good appearance and no oxidative discoloration during storage (corrosion protection, rust prevention);
(2) Good surface propensity of being wetted by solder (solder wettability);
(3) No oxidative discoloration upon high-temperature heating (thermal oxidation resistance); and
(4) Good adhesion to resist (resist adhesion).
To meet these requirements, varied processes for diverse purposes have hitherto been proposed for the treatment of copper foil for printed circuit boards, respectively of its matt and shiny sides. Paying attention to the protection of copper foil against oxidation or corrosion in particular, the present applicant previously proposed, as Japanese Patent Application Publication No. 33908/1986, a process comprising forming a zinc coating film on the shiny side of a copper foil and then forming a chromium oxidation preventive coating layer of a chromium oxide film. The same applicant also proposed and achieved many favorable results from the formation of a mixed coating film of zinc and/or zinc oxide and chromium oxide by electrolytic zinc-chromium treatment (Japanese Patent Application Publication No. 7077/1983). Furthermore, Patent Application Public Disclosure No. 294490/1990 disclosed another process which consists in forming a chromium oxide film by immersion chromate treatment and then forming a mixed coating film of zinc and/or zinc oxide and chromium oxide by said electrolytic zinc-chromium treatment for the purpose of preventing black spotting on copper foil which may be caused when it is exposed to hot, humid conditions for a long time period.
Recently, of among the properties required of the shiny side of copper foil, adhesion to resists, such as plating resist and etching resist, has become particularly important along with thermal oxidation resistance. Good resist adhesion to copper foil is a key to enhanced etching precision to keep up with the tendency toward finer patterns for printed circuits. If resist adhesion is inadequate, infiltration of the etchant such as hydrochloric acid into the region under the resist film would render it difficult to form fine circuit patterns having a very narrow width.
In the procedure of resist printing, it has been customary to mechanically polish the copper foil prior to printing so as to ensure good adhesion of resist to the copper foil.
More recently, for the saving of labor for the process and with the thinning of copper foil for higher density printed circuit boards than before, the trend is toward the omission of the mechanical polishing for enhanced resist adhesion. On the other hand, as noted above, growing importance is attached to the resist adhesion in view of the finer circuit patterns of recent and future printed circuits. Any countermeasure therefore is required which would improve the resist adhesion to thin copper foils without an increase in the number of process steps needed. Needless to say, the countermeasure should not impair not only the other properties the shiny side must have, such as thermal oxidation resistance and solder wettability, but also the many requisites of the matt side, including peel strength and resistance to hydrochloric acid attack.